The invention relates to a Faraday rotator for a Faraday isolator, namely such a Faraday isolator with an input polarizer, with an output polarizer, with a roller-shaped optical crystal that is arranged therebetween symmetrical to its axis of symmetry, with a right hollow cylinder that surrounds this and is made of a permanent magnetic material that is axially magnetized and the magnetic field of which extends in the hollow space approximately parallel to the axis of symmetry that runs in only one direction from the north pole to the south pole, and with terminal magnets, attached to each of the two end faces in the plane perpendicular to the y- and z-directions of the axis of symmetry, that are embodied as hollow vertical cylinders and have a through-aperture in the extension of the axis of symmetry.
Faraday isolators, also called optical isolators, have the object of permitting a laser beam to pass in only one direction. For this, it has an optical rotator, also called a Faraday rotator, polarizers being mounted on both the input and output thereof, and their direction of polarization to one another forms a 45° angle. In general the Faraday rotator comprises a roller-shaped crystal made of a magnetooptical material (for instance TGG). The crystal is surrounded by a hollow right cylinder made of a permanent magnetic material that generates a magnetic field that runs along the axis of symmetry of the crystal. The Faraday effect occurs in that the direction of polarization of the incoming laser beam is rotated by a certain angle when it passes through the crystal. The direction of rotation of the polarization direction is independent of the propagation direction of the laser beam. The size of the angle of rotation is a function of one of the characteristic constants for the material of the optical crystal. This itself is a function of the wavelength of the laser beam. The angle of rotation of the direction of polarization during operation is adjusted such that it is approximately 45°. The output polarizer is also arranged rotated about this angle, and in addition transmits the maximum radiation intensity. A beam that runs against the propagation direction passes the output polarizer and is rotated 45° (in the same direction), that is, a total of 90°, by the Faraday rotator, so that high quenching, also called extinction, is effected for the returning laser beam. In order to increase this further to a higher extinction, so-called two- or even multi-stage Faraday isolators are used in which the extinction is further enhanced.
Such a generic Faraday isolator is known in and of itself. The roller-shaped magnetooptical crystal is surrounded by a right hollow cylinder with a circular cross-section and made of permanent magnetic material that is polarized magnetically in the axial direction. One terminal magnet, in the form of a right hollow cylinder with a circular cross-section, can be connected on either side to the two end surfaces of this hollow cylinder, which are both magnetized parallel to the axis of symmetry of the magnetooptical crystal, that is, also in the axial direction, like the hollow cylinder surrounding the crystal. In addition, the two terminal magnets are magnetized axially in the same direction to one another and with reference to the hollow cylinder opposite the hollow cylinder as central magnet.
Such a generally known Faraday isolator has proved itself. However, a more compact structure is not possible in order to attain the necessary magnetic field strengths in the magnetooptical crystal.
The object of the invention is therefore to embody more compactly a generic Faraday isolator with good homogeneity of the magnetic field strengths.